Diagenetic Fluid and System of Dolomite Reservoir in Huanglong Formation, Eastern Sichuan Basin, China
The dolomite of the Carboniferous Huanglong Formation
(equivalent to Moscovian) is one of the most significant natural gas reservoirs
in Eastern Sichuan Basin. Based on the comprehensive study of the
texture-genetic classification of dolomite, the diagenetic behavior and
evolution, the trace (e.g. Fe, Mn and Sr) and rare earth elements (REE), as
well as the stable isotopes (e.g. C, O and Sr), from the combined
characteristics of basin-scale diagenesis, four subsystems of different diagenetic stages are identified in Huanglong Formation according to the
“hydrological system”, including pore water, compaction brine,
meteoric water and thermobaric water. Three types of diagenetic fluids are
closely related to the development of the dolomite reservoir in each subsystem:
(1) the burial formation brine conserved in the early Sabkha had lower Sr,
strong Eu negative anomalies, slightly negative δ13C and δ18O and
high 87Sr/86Sr ratio. As the burial depth increased, the reducibility and
temperature of the fluid increased gradually.
(2) The strong-oxidizing and
low-temperature meteoric water in open paleo-epidiagenetic stage had the Fe and Mn enrichment, slightly high negative Ce anomalies, negative δ13C and
δ18O and extremely high 87Sr/86Sr ratio.
(3) The strong-reducing and
deep-seated mixed thermal brine in reburial diagenetic stage had Mn enrichment,
Fe depletion, weakly positive Eu anomalies and obviously negative δ13C and
δ18O. The origin and nature of the fluid in each diagenetic system had the
inherited familiarity of development and evolution. And the mechanism of
water-rock reaction, the diagenetic behavior, the products and their combined characteristics
were various. Consequently, we offer some new ideas of the controlling factors
and distribution for the dolomite reservoir development: (1) Facies belts of
the barrier beach and offshore shoal controlled the regional distribution and
the size of the reservoir; (2) Early diagenetic burial dolomitization in
formation brine subsystem was the basis for the reservoir development; (3) The
meteoric water karstification in paleo- epidiagenetic stage expanded the
distribution and the size of the reservoir, and improved the reservoir
qualities; (4) The corrosion in deep subsurface and the tectonic fracturing in
reburial diagenetic subsystem of thermobaric water further improved the
porosity and permeability of the reservoir.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California